Surge is an unstable, pulsating condition that can occur in any compressor that is improperly operated. If the flow rate to the compressor is sufficiently reduced, the compression ratio P.sub.d /P.sub.s where P.sub.d is discharge pressure and P.sub.s is suction pressure, will increase. If the flow decreases too much, the compression ratio will increase to a point where flow reversal occurs inside the compressor which is called "surge." Surge is usually evidenced by an audible boom, piping vibrations, and pressure pulsations. Operation under surge conditions should be avoided because surge can cause thrust bearing failure which can result in rubbing and severe damage to the compressor internals. Overheating due to prolonged surging also causes damage.
The conditions under which a compressor will experience surge is shown on families of performance curves calculated for the compressor. The performance curves include among other items a surge line which indicates at what point surging will occur. A control system must be used to determine if the conditions under which the compressor is operating are approaching the surge line. If so, surge can then be prevented by maintaining a minimum flow through the compressor. Maintaining a minimum flow is accomplished by allowing gas to recirculate through an antisurge valve and recycle line from the compressor's discharge to its inlet. For air and other contaminant free gases, the recycle line is sometimes eliminated and the antisurge valve vents gas to the atmosphere to reduce the compression ratio to prevent surge.
Many prior art antisurge control system typically measure and compute the compressor's operating point relative to a surge line that is determined based on conventional performance curves for various conditions using P.sub.d, P.sub.d -P.sub.s, P.sub.d /P.sub.s, polytropic lead, etc. versus volumetric flow rate squared However, for certain applications, for example, a multistage compressor that must handle extreme gas variations, these prior art control systems will usually have significant measurement errors that can result in inefficient compressor operation and/or failure to prevent surge. This is because these prior art systems do not take into account variation of factors such as the molecular weight of the gas, temperature, compressor speed and/or pressure. Variations in molecular weight are of particular importance where the compressor is to handle different gases with wide variations in molecular weight. The surge line of a compressor determined by conventional methods will vary widely as the molecular weight of the gas changes. This can result in the compressor surging for no apparent reason because the surge line being used to control the compressor becomes incorrect with a shift in the molecular weight of the gas.
Thus, there is a need for an antisurge control system that can prevent surge where variations in molecular weight, pressure and/or temperature will be occurring. There is a need for a method and apparatus for use in an antisurge control system to accurately measure and compute the compressor's operating point relative to its surge point regardless of wide swings in the molecular weight of the gas.
While the molecular weight of the gas at the inlet of the compressor can be measured to monitor changes in molecular weight, such measurements and use of such measurements can be complicated and lacking in sufficient accuracy. Thus, a need exists for a surge control system that is based on parameters that are easily and accurately measured Likewise, a need exists for a method and apparatus for use in an antisurge control system that can use parameters that are easily and accurately measured to compute the compressor's operating point relative to its surge point.